You need need to add some clarity here:
The D750's and most of the D850's never had a thermostat, they have a water flange, that is also the dynamo attachment.
A few of the D850's and B950's had a water flange with a thermostat housing but the motor also had a water pump attached to the gear case cover.
So what motor or application do you need it for, as putting a thermostat housing on a non water pumped motor will fry it in nothing flat!